An electric power steering apparatus energizes a steering mechanism of an automobile or a vehicle by means of a rotational torque of a motor, applies a driving force of the motor to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate an assist torque (steering assist torque), such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty ratios of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a steering wheel 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Tr of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a current command value of an assist (steering assist) command based on a steering torque Tr detected by the torque sensor 10 and a vehicle velocity Vel detected by a vehicle velocity sensor 12, and controls a current supplied to the motor 20 based on a voltage control value E obtained by performing compensation with respect to the steering assist command value. Moreover, it is also possible to receive the vehicle velocity Vel from a controller area network (CAN) or the like.
In such the electric power steering apparatus, for example as disclosed in Japanese Published Unexamined Patent Application No. H8-290778 (Patent Document 1), by means of a robust stabilizing compensator within the control unit 100, a stability of the system and sensitivity characteristics of a road information and a disturbance information are simultaneously designed. However, in such the conventional control unit, since a reaction force during steering in the vicinity of a steering neutral point is small, it is difficult to accurately transmit the road information to a driver due to an influence of friction. Further, in the conventional electric power steering apparatus, it is difficult to set a hysteresis characteristic between a steering angle and a steering force to a characteristic at the same level as a hydraulic power steering apparatus.
As an electric power steering apparatus for dissolving the above problems, there has been proposed the control unit disclosed in Japanese Patent No. 4715212 B2 (Patent Document 2) by the present applicant.
In the control unit disclosed in Patent Document 2, as shown in FIG. 2, the motor 20 for generating the steering assist torque of the steering apparatus is driven by a motor driving section 21, the motor driving section 21 is controlled by the control unit 100 indicated by a dashed-two dotted line, and the steering torque Tr from the torque sensor and the vehicle velocity Vel from the vehicle velocity sensor are inputted into the control unit 100. In the motor 20, a motor inter-terminal voltage Vm and a motor current value i are measured and they are inputted into the control unit 100.
The control unit 100 comprises a torque system control unit 110 indicated by a dashed line that performs a control by using the steering torque Tr and a motor system control unit 120 indicated by a dashed-dotted line that performs a control relating to driving of the motor 20. The torque system control unit 110 comprises an assist amount calculating section 111, a differential control section 112, a yaw rate convergence control section 113, a robust stabilization compensating section 114 and a self-aligning torque (SAT) estimating section 117 and a feedback section 118, and further includes addition sections 116A, 116B and 116C. Further, the motor system control unit 120 comprises a compensating section 121, a disturbance estimating section 122, a motor angular velocity calculating section 123, a motor angular acceleration calculating section 124, a motor characteristic compensating section 125, and addition sections 126A and 126B.
The steering torque Tr is inputted into the assist amount calculating section 111, the differential control section 112, the yaw rate convergence control section 113 and the SAT estimating section 117, and all of them input the vehicle velocity Vel as a parameter. The assist amount calculating section 111 calculates an assist torque amount based on the steering torque Tr. The yaw rate convergence control section 113 inputs the steering torque Tr and a motor angular velocity ω, and brakes a movement that the steering wheel whirls to improve the convergence of yaw of the vehicle. Further, the differential control section 112 enhances a control responsibility in the vicinity of a neutral point of the steering and realizes a smooth steering. Moreover, the SAT estimating section 117 inputs the steering torque Tr, a current command value Iref1 which is a signal obtained in the addition section 116A by adding the output of the differential control section 112 to the output of the assist amount calculating section 111, the motor angular velocity ω calculated by the motor angular velocity calculating section 123 and a motor angular acceleration *ω from the motor angular acceleration calculating section 124 to estimate an SAT, performs signal processing by using the feedback section 118 with respect to the estimated SAT value *SAT and obtains an SAT compensation value *SATc. Then, the compensation is performed by adding the SAT compensation value *SATc to the current command value Iref3 from the robust stabilization compensating section 114 at the addition section 116C so as to provide the steering wheel with suitable road information as a reaction force.
Further, a signal that is obtained in the addition section 116B by adding the output of the yaw rate convergence control section 113 to a signal obtained in the addition section 116A by adding the output of the differential control section 112 to the output of the assist amount calculating section 111, is inputted into the robust stabilization compensating section 114 as a current command value Iref2. For example, the robust stabilization compensating section 114 is a compensating section disclosed in Japanese Published Unexamined Patent Application No. H8-290778 A, removes peak values in a resonance frequency of a resonance system comprised of an inertia element and a spring element that are included in the detected torque, and compensates a phase shift of the resonance frequency that disturbs the responsibility and the stability of the control system. By adding the output *SATc of the feedback section 118 to the output Iref3 of the robust stabilization compensating section 114 in the addition section 116C, a current command value Iref4 capable of transmitting the road information to the steering wheel as the reaction force, is obtained.
Moreover, the motor angular velocity calculating section 123 calculates the motor angular velocity ω based on the motor inter-terminal voltage Vm and the motor current value i, and the motor angular velocity ω is inputted into the motor angular acceleration calculating section 124, the yaw rate convergence control section 113 and the SAT estimating section 117. The motor angular acceleration calculating section 124 calculates the motor angular acceleration *ω based on the inputted motor angular velocity ω, and the calculated motor angular acceleration *ω is inputted into the motor characteristic compensating section 125. The output Ic of the motor characteristic compensating section 125 is added with the current command value Iref4 in the addition sections 126A, the current command value Iref5 being the addition result is inputted into the compensating section 121 comprising a proportional (P) control, a differential (D) control and so on. A current command value Iref6 compensated in the compensating section 121 is added with an output of the disturbance estimating section 122 in the addition section 126B, and a current command value Iref7 being the addition result is inputted into the motor driving section 21 and the disturbance estimating section 122. The disturbance estimating section 122 is an apparatus disclosed in Japanese Published Unexamined Patent Application No. H8-310417 A, is capable of maintaining a desired motor control characteristic in an output reference of the control system based on the current command value Iref7 and the detected motor current value i, and does not lose the stability of the control system.
Further, the SAT estimating section 117 inputs the steering torque Tr, the motor angular velocity ω, the motor angular acceleration *ω and the current command value (assist amount calculation result) and estimates an SAT by using a conventionally well-known method. The estimated SAT value *SAT is processed in the feedback section 118 and the processed SAT compensation value *SATc is inputted into the addition section 116C. For example, the configuration of the feedback section 118 is shown in FIG. 3, and the feedback section 118 comprises a vehicle velocity sensitive high pass filter (HPF) 118-1 and a vehicle velocity sensitive gain section 118-3. The high pass filter 118-1 inputs the estimated SAT value *SAT and outputs a high frequency component, and the gain section 118-3 multiplies with a gain G.
In FIG. 3, “J” indicates inertia, “Fr” friction (static friction) and “Tm” an assist torque.